1. Field of the Invention
The present invention relates to compositions and methods for inhibiting the formation, deposition and adherence of alkaline earth metal scale deposits, especially calcium sulfite (CaSO.sub.3) scale deposits, on metallic surfaces of aqueous systems, especially under conditions of high pH and high calcite concentrations, e.g., those found in lime kiln flue gas scrubber systems, where those compositions comprise polyether polyamino methylene phosphonates.
Generally, calcium sulfite scale deposits are incrustation coatings which accumulate on the metallic surfaces of a water-carrying system through a number of different causes.
Various industrial and commercial water- carrying systems are subject to calcium sulfite scale formation problems. Calcium sulfite scale is of particular concern in paper mill process waters, particularly in lime kiln flue gas scrubber systems, such as those found in sulfate pulp mills, particularly where severe conditions including high pH and high calcium sulfite concentrations are encountered.
Lime kiln flue gas scrubbers serve two purposes; the removal of pollutants, such as SO.sub.2, from the flue gas, and the reclamation of process chemicals for recycling to the pulping process. The scrubber removes both particulates and gases from the lime kiln exhausts. The particulates or dust comprise, for example, calcium oxide and calcium carbonate, while the gases being scrubbed comprise, for example, carbon dioxide and the oxidized TRS gases such as SO.sub.2. The scrubber receives a spray of high pressure filtered water, which water removes the particulates, gases, and heat from the flue gas. The following chemical reactions are typical in the lime kiln flue gas scrubber water system: EQU CaO+SO.sub.2 .fwdarw.CaSO.sub.3 EQU CaO+CO.sub.2 .fwdarw.CaCO.sub.3 EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2 +heat
The salts produced in the above reactions, calcium sulfite and calcium carbonate, as well as calcium sulfate when excess oxygen is present, tend to precipitate on the surfaces of the flue gas scrubber system equipment.
The water employed in these systems ordinarily will contain a number of dissolved salts, and the alkaline earth metal cation calcium is usually prevalent, as is the anion carbonate. The combination product of calcium cation and sulfite, sulfate, and/or carbonate anion will precipitate from the water in which they are carried to form scale deposits when the concentration of the anion and cation comprising the reaction product, i.e., calcium sulfite, exceeds the solubility of the reaction product itself. Thus, when the concentrations of calcium ion and sulfite ion exceed the solubility of the calcium sulfite reaction product, a solid phase of calcium sulfite will form as a precipitate. Precipitation of the reaction product will continue until the solubility product concentrations of the constituent ions are no longer exceeded.
Numerous factors may be responsible for producing a condition of supersaturation for the reaction product calcium sulfite. Among such factors are changes in the pH of the water system, evaporation of the water phase, rate of heat transfer, amount of dissolved solids, and changes in the temperature or pressure of the system. For example, an increase in temperature of the aqueous system decreases the solubility of calcium sulfite and increases the evaporation rate of the water phase, increasing the concentration of undissolved calcium sulfite available for scale formation.
For lime kiln flue gas scrubber systems, deposition occurs on the inside walls of the scrubber itself, scrubber water sump tank, agitators, pumps, and related piping. The mechanism of scale formation is apparently one of crystallization of scale-forming salts from solution, primarily from evaporation of the scrubber water upon contact with hot flue gas, increasing the concentration of scale forming salts in the remaining scrubber water collected in the sump tank. Precipitation is also favored due to the heat transfer from the hot flue gases to the scrubber water, because of the inverse solubility relationship of calcium sulfite. As a result, the solubility of the scale-forming calcium sulfite salt reaction product is exceeded and crystallization of calcium sulfite scale results directly on the hot scrubber surface, particularly at the inlet and outlet of the scrubber.
The formation of calcium salt scale deposits poses a serious problem in a number of regards. Calcium salt scale formation facilitates corrosive processes, and a substantial calcium salt scale deposit will interfere materially with fluid flow. Consequently, calcium salt scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.
Although the present invention is directed primarily to preventing or inhibiting the deposition of calcium sulfite scale on lime kiln flue gas scrubber surfaces, the most prevalent type of scale deposit experienced on these surfaces, it is also applicable to inhibiting the deposition of other types of alkaline earth metal scales, on other surfaces exposed to aqueous systems, especially where those are associated with calcium sulfite scale under the severe conditions described herein. For example, most industrial and commercial water contains alkaline earth metal cations, such as calcium and magnesium, etc., and several anions such a bicarbonate, carbonate, and phosphate. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. These precipitates are alkaline earth metal scales. Thus, as used herein, by "alkaline earth metal scales" and "scale forming-salts" is meant scales formed by true alkaline earth metals such as calcium and barium, but also salts of other metals such as magnesium and sodium, including, but not limited to, calcium sulfite, calcium sulfate, calcium carbonate, magnesium carbonate, burkeite (Na.sub.2 CO.sub.3.2Na.sub.2 SO.sub.4) and calcium phosphate. These scales form frequently in the tubes of heat exchangers and on other heat exchange surfaces, such as those in cooling towers. Particular systems or applications areas where severe conditions lead to exceptional buildup of calcium carbonate and related scales in addition to cycled up cooling towers, include reverse osmosis systems, sugar refining evaporators, and certain types of gas scrubbers.
The polyether polyamino methylene phosphonates of the present invention are used in the same range of amounts as threshold inhibitors in the scale inhibition method of the present invention, rather than as sequestering or chelating agents, although the compositions of the present invention have dispersant and crystal modification properties as well and significantly reduce the adherency of any scale deposit which is formed, facilitating its easy removal.
Scale-forming compounds can be prevented from precipitating by inactivating their cations with chelating or sequestering agents, so that the solubility of their reaction products is not exceeded. Generally, this requires many times as much chelating or sequestering agent as cation, since chelation is a stoichiometric reaction, and these amounts are not always desirable or economical. However, several decades ago, it was discovered that certain inorganic polyphosphates would prevent such precipitation when added in amounts far less than the concentrations needed for sequestering or chelating.
When a precipitation inhibitor is present in a potentially scale-forming system at a markedly lower concentration than that required for sequestering the scale-forming cation (stoichiometric), it is said to be present in "threshold" amounts. See, for example, Hatch and Rice, Indust. Eng. Chem., 31, 51-53 (1939); Reitemeier and Buehrer, J. Phys. Chem., 44 (5), 535-536 (1940); Fink and Richardson U.S. Pat. No. 2,358,222; and Hatch, U.S. Pat. No. 2,539,305.
Similarly, anionic and cationic polymers can be used as dispersant in accordance with methods known in the art, but the dosage levels necessary to achieve dispersion are in the range of 0.5-1% by weight of the system being treated, which is many orders of magnitude higher that the dosage levels used for the compositions of the present invention. Thus, it is a unique aspect of the present invention that it is possible to achieve essentially nonadherent scale using only threshold inhibitor dosage levels of the composition of the present invention
Recently, attention has been focused on controlling scaling under severe conditions, where conventional treatments such as those described above do not provide complete scale control. For example, the conventional treatments, such as use of polyacrylates or HEDP provide good calcium sulfate scale inhibition in the pH range of 3.0-7.0, but at higher pH ranges, such as pH 10 and higher, ranges typical for lime kiln scrubber water, these treatments form calcium salts and become ineffective.
Current technology in scale control can be used to inhibit CaCo.sub.3 scale up to 100 to 120 times calcite saturation i.e. a water containing Ca.sub.+ and CO.sub.3.sup.2- present at 100 times (100.times.) the solubility limit of calcium as calcite (calcite is the most common crystalline form of calcium carbonate). However, what is desired are inhibitors effective in greater than 150.times. water, especially in greater than 250.times. water, and more especially in greater than 300.times. water, i.e., whether the calcite ions can be prevented from precipitating as calcium carbonate scale using substoichiometric amounts of an inhibitor. As used herein with respect to calcium saturation, the designation "X" when preceded by a numeral "Y," thus "YX," means the water being treated contains calcium in a concentration Y times the solubility limit of the particular calcium salt of interest for that particular water, taking into account the water temperature, pH, and any other variable known to those of ordinary skill in the art to affect calcium salt saturation levels in water. The compositions of the present invention are especially useful under severe conditions characterized by a calcite saturation level of 150.times. and above, especially 250.times. and above, and more especially 300.times. and above, as defined in the paragraph immediately below.
Severity of the scaling tendency of a water sample is measured using the saturation index, which may be derived in accordance with the following equation: ##EQU1## where SI is the saturation index for calcium carbonate, [Ca.sup.2+ ] is the concentration of free calcium ions, [CO.sub.3.sup.2- ] is the concentration of free carbonate ions, and .sup.K spCaCO.sub.3 is the conditional solubility product constant for CaCO.sub.3. All of the quantities on the right side of the above equation are adjusted for pH, temperature and ionic strength.
Calculation and use of the saturation index, and generation of the data from which it is derived, are matter within the skill of the art. See, for example, Critical Stability Constants, Vol. 4: "Inorganic Complexes", Smith & Mantell (1976), Plenum Press; and Aquatic Chemistry, Chap. 5, 2nd ed., Stumm & Morgan (1981), Wiley & Sons.
In the case of calcium sulfite, as most other scaling salts, concentration of the scaling salt in the aqueous system dictates the degree to which scaling may occur. The effect of concentration is defined by: EQU K.sub.sp =[Ca][SO.sub.3 ]
where
K.sub.sp =solubility constant PA1 [Ca]=concentration of Ca PA1 [SO.sub.3 ]=concentration of SO.sub.3
If the product of the two concentrations exceeds the solubility constant, K.sub.sp, then a driving force for the precipitation of calcium sulfite and scaling is present. This driving force can be expressed by the product of the concentrations divided by K.sub.sp : ##EQU2##
Thus, a driving force of greater than 1 favors precipitation and scaling.
Another characteristic feature of the severe conditions under which the present invention is especially useful is high pH, i.e., a pH of 8.5 and higher, particularly a pH of 9 or 10 or even higher. A related feature of such severe conditions is high alkalinity.
One of the particular advantages of the scale inhibiting compositions of the present invention is the exceptional calcium tolerances which they exhibit. Calcium tolerance is a measure of a chemical compound's ability to remain soluble in the presence of calcium ions (Ca.sup.2+). One of the parameters of scale control under severe conditions is pH. As pH increases, calcium tolerance decreases rapidly for traditional CaCo.sub.3 threshold inhibitors, e.g., 1-hydroxy ethylidene 1,1-diphosphonic acid (HEDP) and amino tri (methylene phosponic acid) (AMP). These inhibitors precipitate with calcium at alkaline pH's, rendering them useless as threshold scale inhibitors. While it is common practice to use an acid feed to the water of, e.g., a cooling tower system in order to lower pH and thus avoid the calcium tolerance problem for conventional inhibits, the danger to handlers which such acid feeding poses makes it all the more important to find scale inhibitors which operate at high pH's.
2. Brief Description of the Prior Art
Early efforts to reduce scale formation in water-carrying systems employed compounds such as tannins, modified lignins, algins, and other similar materials. Chelating or sequestering agents have also been employed to prevent precipitation or crystallization of scale-forming calcium carbonate. Another type of agent which has been actively explored heretofore as a calcium carbonate scale inhibiting material is the threshold active inhibitor. Such materials are effective as scale inhibitors in amounts considerably less than that stoichiometrically required, and this amount, as already mentioned, is termed the threshold amount. Inorganic polyphosphates have long been used as such threshold active inhibitors. For examples of such materials, see Fink, U.S. Pat. No. 2,358,222; Hatch, U.S. Pat. No. 2,359,305; and Ralston, U.S. Pat. No. 3,434,969.
Certain water soluble polymers, including groups derived from acrylamide and acrylic acid have been used to condition water containing scale-forming calcium carbonate. For example, see U.S. Pat. Nos. 2,783,200; 3,514,476; 2,980,610; 3,285,886; 3,463,730; 3,518,204; 3,928,196; 3,965,027; and 4,936,987. In particular, there has been employed anionic polyelectrolytes such as polyacrylates, polymaleic anhydrides, copolymers of acrylates and sulfonates, and polymers of sulfonated styrenes. See, for example U.S. Pat. Nos. 4,640,793; 4,650,591; 4,457,847; and 4,671,888. When used as threshold alkaline earth metal scale inhibitors, however, large dosages of these polymers are required, which in turn increases operating costs.
While various polycarboxylates, including polyacrylic acid, have been used as scale inhibiting agents, as described above, no similar use has been made of polycationic agents, apparently because of the difference in electronic charge and the conventional theories of the mechanisms of action for polymeric threshold inhibitors and dispersant.
While polyether polyamino methylene phosphonates of the type which comprise an active ingredient of the compositions of the present invention are known, their use for the control of alkaline earth metal scale, particularly calcium sulfite scale, under severe conditions which include elevated pH and high calcium saturation levels, has not heretofore been suggested.
For example, U.S. Pat. No. 4,080,375 discloses methylene phosphonates of amino-terminated oxyalkylates for use as scale inhibitors, but these compositions are not the same as those of the present invention, nor is there any suggestion that such compositions would be useful under severe conditions as defined herein, where phosphonates such as HEDP and AMP give poor results. U.S. Pat. No. 4,931,189 discloses aminomethylene phosphonates of the type used in the method of the present invention, but for inhibiting oil field scale formation involving a high brine environment susceptible to gypsum or barite scale formation. Such use in no way suggests the control of scale under the severe conditions described herein under which the compositions and methods of the present invention operate with surprising success.
A particular phosphonate which has been marketed for scale control, but apparently not suggested for use under the severe conditions defined herein, is ethanolamine, N,N-dimethylene phosphonic acid, sold under such trademarks as WAYPLEX 61-A and BRIQUEST 221-50A, and described in EP-A00 384 779; U.S. Pat. No. 2,917,528; and U.S. Pat. No. 2,964,549.
U.S. Pat. No. 5,338,477 discloses the use of a polyether polyamino methylene phosphonate of the type which comprises an active ingredient of the present invention, but fails to disclose the effectiveness of this inhibitor for treating calcium sulfite.